Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a PoE system that involves delivering power over unshielded twisted-pair wiring from a PSE to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a PoE detection signature.
The IEEE 802.3af standard requires that PSE ports must be electrically isolated from chassis. Hence, some circuit arrangement must be provided to electrically isolate PSE port control circuitry that controls operations of the PSE with respect to supplying power to PDs linked to particular PSE ports. In particular, the PSE port control circuitry should be isolated from sources of any digital signals supplied to the PSE port control circuitry, circuits that receive any digital signals supplied by the PSE port control circuitry, and/or from power sources needed to operate the PSE port control circuitry.
The conventional approach involves optocouplers used to isolate the digital control interface, and a switching power supply used to provide power to the PSE port control circuitry. For example, FIG. 1 shows a conventional PoE system including PSE port control circuitry 10 isolated from a plurality of digital control and/or status signals using optocouplers 12. Power to run at least part of the PSE port control circuitry 10 is provided by an isolated switching-mode power supply 14. A main power supply 16, typically −48 Vdc, provides power that the PSE port sends to a respective PD.
FIG. 2 shows another example of a conventional PoE system, in which the power to operate the PSE port control circuitry 10 is derived from the main power supply 16 via a non-isolated switching-mode power supply 18.
Among disadvantages of the conventional PSE isolation systems is their high cost that results in the significant increase of the PSE per-port cost due to the use of the optocouplers and switching power supply. Also, the conventional systems suffer from a high level of switching noise caused by the switching power supply, and occupy a very large board space because optocouplers and some components of the switching power supply require a substantial room.
Therefore, it would be desirable to provide PSE isolation without optocouplers and switching power supplies.